The present invention relates to a multipolar-magnetized cylindrical permanent magnet to be used as a rotor of a permanent magnet motor or a synchronous motor, such as servomotors and spindle motors, and further relates to a permanent magnet motor including the rotor. More particularly, the invention relates to a multipolar-magnetized cylindrical permanent magnet having magnetic anisotropy in a single diametrical direction, or in a single direction perpendicular to the axis of the cylindrical magnet, as well as to a permanent magnet motor including the magnet as the rotor.
As is well known, permanent magnets having magnetic anisotropy, i.e. permanent magnets capable of being more easily magnetized in a specific direction than in other directions, are widely employed as a part of loudspeakers, electric motors, metering instruments and other electric apparatuses. Such an anisotropic permanent magnet is prepared from a permanent magnet material having crystalline magnetic anisotropy, such as certain hard ferrites and rare earth element-containing alloys. The material is pulverized into a powder of fine particles, followed by compression molding of the powder within a magnetic field (referred to as "in-field molding" hereinafter) to provide a powder compact which is followed by sintering of the powder compact. In the in-field molding of the magnetic powder, the magnetic particles are each oriented relative to the easy magnetization axis of the magnet crystallites as a consequence of the magnetic field applied, so that the resultant sintered magnet also has magnetic anisotropy in the direction of the magnetic field applied to the powder under compression during the in-field molding.
The direction of the magnetic field in the in-field molding of the magnetically anisotropic magnetic particles can be either perpendicular or parallel to the direction of compression for the molding. For example, the anisotropic direction, i.e. the most easily magnetizable direction, of a cylindrical permanent magnet prepared from a powder of a rare earth-based magnetic alloy can be either in parallel to the axial direction of the cylindrical form or in a radial direction perpendicular to the axial direction. Cylindrical rare earth permanent magnets having a radial anisotropic direction are employed as rotors in various types of permanent magnet motors such as AC servomotors, DC brushless motors and the like because of the advantages in that they can be freely magnetized in the axial direction, and no reinforcement is required for assembling unit magnets, as is required in the assemblage of segment magnets. In recent years, a radially anisotropic cylindrical permanent magnet having an increased height or dimension in its axial direction has been desired to meet the needs associated with the expansion of the application fields of permanent magnet motors.
A cylindrical permanent magnet having radial anisotropy is prepared usually by the method of in-field molding, or by the method of backward extrusion molding of the magnet powder. In the in-field molding method, while the magnetic alloy powder in a metal mold is compressed in the axial direction of the cylinder, a magnetic field is applied to the powder under compression in a radial direction through cores at each of the opposite ends of the cylinder. Accordingly, the height, i.e. the dimension in the axial direction of the cylinder, of a radially anisotropic cylindrical magnet is limited by the dimensions or shape of the cores, so that a radially anisotropic cylindrical magnet of an increased height can be prepared only with great difficulties. This method is also not productive because only one molded body can be obtained in a single molding operation using a single molding press. The method of backward extrusion molding is also disadvantageous due to the high cost for the preparation of molded bodies, in that the method requires a large and complicated, and hence very expensive, molding machine. Also, the yield of acceptable molded bodies is relatively low. This situation naturally increases the cost of permanent magnet motors using an expensive multi-radially anisotropic cylindrical permanent magnet as the rotor.
Even without using a multi-radially anisotropic cylindrical permanent magnet, a high-performance cylindrical magnet to be used as a rotor in a permanent magnet motor could be obtained when multipolar magnetization of a cylindrical permanent magnet is accomplished with a sufficiently high magnetic flux density on its surface, and with little variation of the magnetic flux densities among the magnetic poles. In this regard, a method is proposed in the papers of Electricity Society, Magnetics Group MAG-85-120 (1985), according to which a cylindrical magnet having magnetic orientation in a single direction perpendicular to the cylinder axis is prepared by using an in-field molding press under application of a magnetic field in the direction perpendicular to the direction of compression (referred to as a diametrically oriented cylindrical permanent magnet hereinafter). The magnet is provided with multipolar magnetization so that a multipolar cylindrical permanent magnet to serve as a rotor in a permanent magnet motor can be obtained without using an expensive multiradially anisotropic magnet.
The above mentioned cylindrical permanent magnet, which is magnetically oriented in a single direction perpendicular to the cylinder axis (referred to as a diametrically oriented cylindrical magnet), may have an increased height of 50 mm or even larger, if permitted by the dimensions of the cavity of the metal mold, and if a multi-stage molding method can be undertaken. Thus, a plurality of diametrically oriented cylindrical magnets can be obtained by a single operation of compression molding using a multi-cavity metal mold at low costs. Such a diametrically oriented multipolar cylindrical permanent magnet can be employed in place of expensive multi-radially anisotropic magnets as a rotor in permanent magnet motors.
Though possible in principle, the above mentioned diametrically oriented cylindrical permanent magnets are practically infeasible as rotors of permanent magnet motors due to the irregular distribution of magnetic flux density around the circumferential surface of the cylindrical permanent magnet. That is, the magnetic flux density is high on the magnetic pole at or in the vicinity of the direction of the diametrical orientation axis, and is low on the magnetic pole at or in the vicinity of the direction perpendicular to the diametrical orientation axis. This results in the rotational torque of the motor constructed with the rotor being necessarily uneven around the rotation axis, corresponding to uneven or irregular distribution of the magnetic flux density.